5-Hydroxymethyl furfural (HMF) is an alternative, non-petroleum precursor which can be used as a building block chemical for producing various high-volume and value-added organic chemicals. These chemicals include 2,5-furandicarboxylic acid (FDCA) which can serve as a precursor in the polymer industry[1], and 2,5-dimethylfuran (DMF) which can be used as a liquid transportation fuel.[2] DMF can also be used to produce p-xylene via cycloaddition with ethylene combined with dehydration over acidic zeolites and acidic oxides.[3] Alamillo et al. have shown quantitative yields of 2,5-di-hydroxy-methyl-tetrahydrofuran (DHMTHF) from HMF with ruthenium-supported oxide catalysts.[4]
HMF is produced conventionally from glucose (in low yields) or fructose (in high yields) by a triple dehydration step with mineral acids in water.[5] It would be highly desirable to be able to produce HMF from cellulose, which is a more abundant and lower value feedstock than fructose. However, in aqueous systems, HMF is only produced in low yields (between 8 to 21%) from cellulose because of miscibility limitations and undesired formation of humins.[6] HMF production is maximized at relatively high temperatures (200-300° C.) and short reaction times (on the order of seconds or minutes). In aqueous systems, HMF is readily converted to formic acid and levulinic acid. The latter compound is also a versatile, bio-based platform chemical.[7] 
The use of ionic liquids (ILs) as solvents for HMF production has been proposed due to the solvation capabilities of the ILs. A HMF yield of 51% from fructose was obtained by Li et al. when a high concentration of feed (67 wt %) was used in 1-butyl-3-methylimidazolium chloride.[8] Binder and Raines developed a process to convert lignocellulosic biomass to HMF using N,N-dimethylacetamide (DMA) containing lithium chloride as a solvent.[9] HMF yields of up to 54% were obtained with 1-ethyl-3-methylimidazolium chloride as an additive and a mixture of CrCl2/HCl as the catalyst. Rinaldi et al. showed that solid acid catalysts can be used in 1-butyl-3-methylimidazolium chloride to selectively depolymerize cellulose to produce glucose and HMF.[10] Zhang and co-workers have reported HMF yields of 55% from cellulose with a mixture of CuCl2 and CrCl2 dissolved in 1-ethyl-3-methylimidazolium chlorid at relatively low temperatures.[11] A comprehensive review covering the process chemistry of HMF production from various feedstocks is given by van Putten et al.[12]
Significant challenges hinder the industrial use of ILs for production of HMF. Due to their high costs, quantitative recovery and recycling of ILs (at least 98%) is necessary to make the process economically attractive.[13] Relative low cellulose solubility (10-15 wt %) in ILs[14], high viscosity, and high toxicity of ILs are also impeding factors.[15] Thermal and chemical stabilities of ILs are also in question, as new compounds have been detected derived from side reactions between HMF and imidazolium-based ILs.[16] Extensive work has been reported by Jerome and co-workers to produce HMF from biomass derived feedstock in alternative solvent systems that are comparable with imidazolium-based ILs.[17] Alternative approaches have also been investigated using biphasic reaction systems with organic solvents that can extract the HMF from the aqueous phase before it undergoes further degradation reactions.[18] Phase modifiers (e.g., NaCl) can be added to the aqueous phase to help enhance HMF partitioning into the immiscible organic phase and consequently impede further HMF degradation.[19]
There thus remains a long-felt and unmet need for an easy, fast, and economical method to produce HMF from biomass.